1. Field of the Invention
The present invention relates to a fuel pump adapted to suck in and pressurize a fuel such as gasoline and discharge the pressurized fuel.
2. Discussion of Related Art
A fuel pump has an impeller and a pump casing, as disclosed in Japanese Patent Application Unexamined Publication (KOKAI) No. Hei 7-279881. The impeller has an approximately disk-shaped configuration with a plurality of blade grooves formed serially in a region extending along the outer peripheries of the obverse and reverse sides of the disk-shaped impeller. The impeller is rotated by a driving device such as a motor. The pump casing surrounds the impeller and has a circumferentially extending recess for forming a circumferentially extending flow passage groove between the same and the blade grooves of the impeller. The pump casing further has a suction opening communicating with the upstream end of the recess and a discharge opening communicating with the downstream end of the recess. Further, the pump casing has a circumferential wall forming an inner peripheral surface extending along the outer peripheral surface of the impeller. When the impeller rotates, fuel is sucked into the flow passage groove from the suction opening and pressurized while flowing circumferentially in the flow passage groove. The pressurized fuel is discharged from the discharge opening.
In this case, the size of the clearance between the impeller outer peripheral surface and the pump casing inner peripheral surface has a significant effect on the pump efficiency. The smaller the clearance, the smaller the amount of fuel leakage, and the higher the pump efficiency.
However, the fuel pump is usually used for a long period of time. During use, the bearings supporting the shaft for rotating the impeller unavoidably wear out, causing the center of rotation of the impeller to be displaced gradually by small amounts. For this reason, if the above-described clearance is set excessively small, the impeller outer peripheral surface and the pump casing inner peripheral surface may contact each other when the rotation center of the impeller is displaced, resulting in a failure of the pump operation.
Therefore, the conventional practice is to allow some margin for the clearance between the impeller outer peripheral surface and the pump casing inner peripheral surface so that these peripheral surfaces will not contact each other even if the rotation center of the impeller is displaced as a result of wear of the bearings.